> In a community with such liberal sexual practices, STDs spread more easily, especially in earlier centuries.

You can use a condom. TIL, rubber condoms are a mid-19th century invention; a significant upgrade over sheep gut.

The alternative is called polyfidelity.

> [...] what are Alice and David to each other?

They're called Metamours.

> Anything? Nothing? Is the entire married community a distinct entity?

It's called a polycule.

> Relatedly, how is inheritance handled if such complex spousal organizations are going to be legally allowed?

You write a will.

By the way, inheritance laws are messy already as they are. Try figuring out how to reject inheritance (e.g. of debt) in your jurisdiction.

> There are reasons to allow only one of the sexes to have multiple sexual partners/spouses.

Yes, the reason is to reinforce division and oppression. One "side" is underprivileged, the other has to fight each other for supremacy. The stronger few win, everyone else loses. History is littered with examples.

Don't get me wrong, these are all very good questions. But we've figured all of these things out quite a while ago. People do live like that, and form lasting, loving communities. I'd wager that an entire society built on top of that would have no lesser chance at thriving than the one we've been born into.

>You can use a condom.

Sure. Now. But monogamy and polygyny are a little older than condoms.

>Metamours [...] polycule

You're answering rhetorical questions which, incidentally, are not about terminology, but about legal and social mechanics. Knowing what a "metamour" is, says nothing about what the formal and informal responsibilities of the parties involved are or should be with respect to each other. My whole point is that not having to define such relationships and their expectations is a reason to forbid them culturally.

>You write a will.

How did that work before most people knew how to write?

>By the way, inheritance laws are messy already as they are.

That's not an argument in favor of legally legitimizing polycules.

>the reason is to reinforce division and oppression

I mean, I gave several reasons why historically either monogamy or asymmetric polygamy would have been preferred over symmetric polygamy, that have nothing to do with oppression.

>I'd wager that an entire society built on top of that would have no lesser chance at thriving than the one we've been born into.

Sure, maybe. Personally, I'm more of the opinion that cultural features are memetic, and that memes are not uniformly successfully propagated. If monogamous and polygynous societies are more common than polyandrous and polycular societies, it's probably for a reason.